Negative Air Machines: Ducting, Containment & Restoration Selection
A negative air machine is not simply an air scrubber with a stronger fan. The useful decision is whether the equipment can move filtered air through the required ducting while supporting the containment strategy and staying within electrical limits. This page owns that setup-oriented intent; general air-scrubber selection and HEPA filter details are covered elsewhere.
Negative air describes how the equipment is used
In recirculating air-scrubber mode, air is drawn from the work area, filtered and returned to that same area. In a negative-air configuration, filtered air is exhausted out of the contained area so the interior pressure can be lower than the surrounding space when the airflow and containment leakage support that condition.
The machine is therefore one component of the pressure strategy. Containment construction, intentional makeup air, openings, duct restriction, filter loading and exhaust routing all affect the actual pressure differential.
Why free-air CFM is only the first number
Manufacturers commonly advertise airflow in CFM, but negative-air setups add resistance. Filters create pressure drop; ducting creates pressure drop; bends and long runs add resistance. A unit with a published 700 CFM free-air maximum does not necessarily move 700 CFM through every installed duct configuration.
That is where static-pressure information becomes useful. Dri-Eaz publishes 3.0 inches H₂O for the HEPA 700 and DefendAir HEPA 700. AlorAir publishes 2.8 inches H₂O for the HEPA Max 870. Those figures provide more context for a ducted application than a CFM headline alone, even though a full fan curve would be needed to know airflow at every resistance point.
Current ducting examples
| Model | Documented ducting / negative-air detail | Published airflow | Static pressure |
|---|---|---|---|
| Dri-Eaz HEPA 700 | Manufacturer documents negative/positive-pressure setups; 14 in. outlet ring and semi-rigid inlet/outlet ducting options are published separately. | 275–700 CFM | 3.0 in. H₂O |
| Dri-Eaz DefendAir HEPA 700 | 14 in. lay-flat ducting port; manufacturer describes air-scrubber/negative-air use. | 275–700 CFM | 3.0 in. H₂O |
| XPOWER X-2480A | Manufacturer says the lipped outlet can be ducted to channel purified air or create negative air pressure. | 550 CFM | Not published in current normalized record |
| AlorAir HEPA Max 870 | 14 in. intake and 8 in. outlet duct sizes published. | 270–550 CFM | 2.8 in. H₂O |
| AlorAir HEPA Max 970 | 14 in. intake and 8 in. outlet duct sizes published. | 270–750 CFM | 2.8 in. H₂O |
Duct diameter and duct length are part of the equipment decision
A buyer should confirm the manufacturer’s actual inlet/outlet dimensions before assuming an existing duct setup will fit. Dri-Eaz’s current HEPA 700 ducting page publishes 14-inch lay-flat outlet ducting and 14-inch semi-rigid ducting that can fit the inlet or outlet. The same page says not to exceed a 25 ft ducting length for that semi-rigid setup.
AlorAir’s HEPA Max 870 publishes a 14-inch intake and 8-inch outlet. Those are materially different connection sizes from a 14-inch outlet-only workflow, so “both are negative air machines” does not mean the field setup is identical.
Pressure differential depends on the containment, not just the machine
A containment with large uncontrolled openings can leak enough air that a selected fan does not achieve the intended pressure relationship. A tighter containment can require less exhaust airflow to reach the same pressure differential. This is why Restoration Select will not publish a universal “X CFM creates Y pressure” rule.
The machine can be selected for airflow, pressure capability, ducting and electrical characteristics. The actual pressure target and verification method belong to the job plan and applicable professional procedures.
HEPA remains important, but this is not the HEPA page
Negative-air equipment used in restoration commonly includes HEPA filtration so exhausted air passes through fine particulate filtration. The details of HEPA efficiency wording, filter stages, carbon and replacement filters are important, but duplicating that full topic here would make this page compete with HEPA Air Scrubber.
For negative-air selection, confirm the compatible filter configuration and then focus on ducting, pressure capability, airflow control and the containment plan.
Need a shortlist where HEPA + negative air are both verified?
The combined HEPA Negative Air Machine page filters the current exact catalog to products with both fields documented, while this page remains the broader containment/ducting owner.
Variable speed can be useful when balancing containment airflow
Variable-speed equipment gives the operator more control over how much air is exhausted without switching machines. The Dri-Eaz HEPA 700 and DefendAir HEPA 700 both publish variable airflow up to 700 CFM. AlorAir publishes variable 270–550 CFM for the HEPA Max 870 and 270–750 CFM for the HEPA Max 970.
Variable speed does not replace measuring or verifying the containment condition, but it can make one machine more adaptable across different volumes and leakage conditions than a single fixed-airflow unit.
Electrical planning gets harder with multiple containment machines
Negative-air projects can involve more than one scrubber plus dehumidifiers, air movers, lighting and tools. That makes amp draw and manufacturer chain limits important. A machine’s GFCI/daisy-chain outlet is convenient, but it is not permission to ignore total electrical load.
Dri-Eaz currently publishes up to seven HEPA 700 units on low on one 15-amp circuit under its stated conditions, while the DefendAir HEPA 700 limit is up to four on low. The difference is exactly why chain limits belong in exact product records rather than a generic negative-air checklist.
When to choose a compact unit vs a higher-airflow unit
A compact XPOWER X-2480A at 23.6 lb can be easier to carry into small containments and upper floors. A HEPA Max 970 or HEPA 700 offers more maximum airflow. A DefendAir 400 may fit a smaller-loss fleet where 400 CFM per unit is enough and multiple units are intentionally distributed.
The selection should therefore begin with required airflow and ducting, then consider transport, power and filter support. “Largest CFM” is not a complete negative-air strategy.
Shopping paths for ductable scrubbers
The Recommended Restoration Equipment page compares these machines in broader buying-path context. For ducted work, the current featured HEPA 700, HEPA Max 970, HEPA Max 870 and XPOWER X-2480A each have different airflow/weight/ducting tradeoffs.
Related air-scrubber decisions
Use the page that matches the decision you are actually making. These pages intentionally separate broad selection, filtration, mold-remediation context, containment/negative air, commercial fleet considerations and consumer-air-cleaner comparisons.
- Air Scrubber
- HEPA Air Scrubber
- Air Scrubber for Mold
- Negative Air Machine — current page
- Commercial Air Scrubber
- Air Scrubber vs Air Purifier
- Recommended Equipment
Use the live air-scrubber tools
Size the airflow first when needed, use Finder to build a requirement-based shortlist, then compare exact normalized rows without another ranking layer.
Primary references for this page
- Dri-Eaz HEPA 700 Owner’s Manual ↗Current manufacturer manual describing negative- and positive-pressure filtration setups and operating cautions.
- Dri-Eaz HEPA 700 ducting ↗Manufacturer duct sizes and published maximum semi-rigid duct length for HEPA 700 containment use.
- Dri-Eaz HEPA 700 ↗Current airflow, static pressure, electrical load and daisy-chain information.
- XPOWER X-2480A ↗Manufacturer wording for ducted filtered air / negative air pressure and current airflow/electrical specifications.
- AlorAir PureAiro HEPA Max 870 ↗Current inlet/outlet duct sizes, variable airflow and static-pressure specification.
Frequently asked questions
What is a negative air machine?
A negative air machine is filtration equipment used so air is exhausted from a contained space through filters, helping maintain lower pressure inside the containment than outside when the setup and airflow are appropriate.
Is a negative air machine different from an air scrubber?
Often the same equipment class can perform both roles. 'Air scrubber' usually describes recirculating filtration, while 'negative air' describes a ducted containment setup. The manufacturer must support the required ducting/configuration.
Does more CFM always create better negative pressure?
No. Pressure differential depends on containment leakage, duct resistance, filters, makeup air and the actual installed airflow. Free-air CFM alone does not guarantee a target pressure.
Where should filtered exhaust air go?
That is a job-specific setup decision. Follow the remediation plan, exact equipment manual and applicable professional requirements. Restoration Select does not prescribe a universal discharge location.
Can I daisy-chain several negative air machines?
Only within the exact manufacturer's published electrical limits and the jobsite circuit plan. Daisy-chain capability is model-specific and does not replace electrical load calculations.